The present invention relates to a system and method for diagnosing errors on asymmetric digital subscriber line (ADSL) subscriber loops, wherein a successful connection cannot be achieved.
Remote access and retrieval of data is becoming increasingly popular in data communication. The proliferation of the Internet has provided a vast network of information that is available to the general public. As the Internet grows and technology advances, this information is becoming increasingly voluminous and the details are become increasingly intricate. What used to comprise mainly text information has grown to include still and moving images as well as sound. The increase in the volume of information to be transferred has presented a need for a high-speed Internet connection, since traditional telephone modems communicate at speeds to slow for efficient communication.
One proposal for high-speed communication is the introduction of Digital Subscriber Line (DSL) technology. One of the most attractive features of DSL is that it is implemented using an infrastructure that already exists. DSL shares copper twisted pair lines typically used for telephone communication. However, only a small portion of the available bandwidth of the twisted pair line (0 to 4 kHz) is used for Plain Old Telephone Service (POTS). DSL takes advantage of the available frequency spectrum from 4 kHz to approximately 1.1 MHz for transmitting data.
Asymmetric DSL (ADSL) is currently the most practical form of DSL technology, and therefore the most widely implemented. ADSL is asymmetric in that its downstream (to a subscriber) capacity is larger than its upstream (from the subscriber) capacity. Typically, a Discrete Multi-tone (DMT) scheme is used. The spectrum from 4 kHz to 1.1 MHz is divided into 256 sub-channels, or tones, each having a bandwidth of 4.3125 kHz. Each sub-channel uses Quadrature Amplitude Modulation (QAM) to carry 2 to 15 bits/QAM symbol.
In accordance with ADSL standard ITU G.992.2, several phases occur in order to initialize a communication link. These phases include handshaking, transceiver training, channel analysis and exchange.
Handshaking is used for determining the nature and capabilities of communication endpoints (such as an ADSL modem) and for indicating which protocol will be used for the remainder of the initialization. The ADSL modem, or termination unit, at a central office is referred to as an ATU-C. Similarly, the ADSL termination unit at the subscriber, or remote location, is referred to as the ATU-R.
The signalling method used for the handshake interchange is designed to be robust. Biphase shift keying (BPSK) modulation is often used to modulate multiple single-tone sub-carriers, all carrying the same data. Typically, the ATU-C and ATU-R exchange a message containing information about the endpoint type, frequency range, and number of DMT sub-carriers supported.
During transceiver training, the transceivers at each end of the line acquire a DMT symbol stream, adjust receiver gain, perform symbol-timing recovery, and train any equalizers. There is an optional echo cancellation training step that can also be performed during this phase.
During channel analysis, the transceivers exchange capability information and perform detailed channel characterization. Both the ATU-R and ATU-C attempt to measure specific channel characteristics such as unusable sub-carriers, loop attenuation on a per sub-carrier basis, SNRs, and any other channel impairments that would affect the potential transmitted bit rates. Based on the discovered channel characteristics, the ATU-C makes the first offer of the overall bit rates and coding overhead that will be used for the connection.
The exchange phase sets the final overall transmission rates in both the upstream and downstream directions for the connection. These final rates are determined based on calculated channel parameters measured during the channel analysis phase, and are not necessarily the same as the preliminary rates offered during that phase.
Furthermore, the exchange phase sets forward error correction (FEC) and interleaver configurations. Generally, the configurations are close to the optimum bit rate for the channels. Four carriers are used to modulate the bits of the messages, each carrier being loaded with 2 bits using quadrature phase shift key (QPSK) modulation.
Since the ATU-C controls data rates, if the ATU-R cannot support any of the offered rates, both terminals will return to the beginning of the initialization process. Otherwise the ATU-R responds with the rate it can support.
However, sometimes the transceivers may not be capable of successfully reaching a data transfer stage, referred to as SHOWTIME, due to poor channel conditions. In this case there is a need for a system to be able to diagnose the problems preventing the transceivers from initializing so that they can be corrected or avoided.
It is an object of the present invention to obviate or mitigate at least some of the above mentioned disadvantages.